Thanks to Mike Marcelais on TA for providing a breakdown of the XP system.
After you have gotten all of the other achievements, boosting on multiplayer is the easiest. You are guaranteed 25 XP per game no matter what and do not have to worry about any nasty 30-second ads. Essentially you can just keep starting multiplayer games and making a single match and let the clock run out. This will earn you roughly 25 XP ever 90 seconds, or 1000 XP per hour. This is roughly the same rate that you will earn while repeatedly playing level 15 of Forest to evolve all the Mythical monsters for the "A World Much Sweeter" achievement. At these rates, it will take you a minimum of 45 hours to complete this achievement and the game.

level 19 and got everything else. If it takes about 90 secs to do a multiplayer level (finding a game, loading, and 60secs gameplay), then i calculate it would take over 26 hours of boosting to get to level 30... nah, don't think i'll bother